Microwave bulk acoustic delay devices are very costly to fabricate. These devices preferably use lithium niobate transducers made of thin wafers in submicron thicknesses metallized on opposite surfaces thereof. The lithium niobate transducer is bonded to a bulk acoustic delay medium such as magnesium aluminate (MgOAl.sub.2 O.sub.3) spinel. Lithium niobate is desirable as a transducer since it has a very strong electro-mechanical coupling coefficient of 0.68. Lithium niobate generates an acoustic wave propagating in the shear mode along the (100) direction in the magnesium aluminate spinel. The spinel is suitable as a delay medium as it has extremely low acoustic attenuation for shear bulk acoustic waves.
However, the fabrication of such devices includes very expensive and costly techniques of bonding, lapping, polishing and ion beam milling the lithium niobate transducer to required sub-micron thicknesses. The surfaces of the lithium niobate and the spinel are polished to flatnesses of better than 1/10 of .lambda. where .lambda. is the wavelength of sodium light. The surfaces of the various layers are parallel in seconds of arc. A detailed description of the fabrication of sub-micron lithium niobate transducers for microwave bulk acoustic delay lines is presented in several articles.
H. C. Huang, J. D. Knox, Z. Turski, R. Wargo and J. J. Hanak, "Fabrication of Sub-micron, LiNbO.sub.3 Transducers for Microwave Acoustic (Bulk) Delay Lines," Applied Physics Letters, Feb. 1974, Vol. 24.
H. C. Huang, J. D. Knox and Z. Turski, "Bulk Acoustic Delay Lines Utilizing Sub-micron Platalet LiNbO.sub.3 Transducers," IEDM Technical Digest, 1973
In utilizing an anisotropic material such as spinel as the bulk delay line substrate, certain restrictions on the propagation direction are present. In one prior art configuration a single transducer serves as both the input and output ports for the device. In this configuration, an input signal is applied to the transducer and the acoustic wave launched is reflected off the oppositely disposed delay device surface to the same transducer. The separation between the input and output signals is accomplished by a microwave circulator having approximately 40 db or less isolation. There are problems present with such devices in that an input signal when applied to the single transducer may be reflected immediately producing a spurious signal feed directly to the detection circuitry. In most applications such spurious signals are undesirable.
In an alternate arrangement, the input and output transducers are disposed on respective opposite ends of the delay substrate. This arrangement is much more costly to fabricate in that two separate transducers are required.